The present invention relates to an induction machine which can be used, for example, as both a starting motor and an electrical generator in an aircraft electrical system.
Power conversion systems, such as those used on aircraft to generate electrical power, typically include a brushless, three-phase synchronous generator which operates in a generating mode in order to convert variable speed motive power supplied by a prime mover, such as an engine, to electrical power. This three-phase synchronous generator can also be operated as a motor in a starting mode in order to convert electrical power supplied by an external electrical power source to motive power that is used to turn the engine and bring the engine up to its self-sustaining speed.
A typical brushless, three-phase synchronous generator includes a permanent magnet generator (PMG), an exciter, and a main generator all mounted on a common drive shaft. During starting, it is known to provide electrical power at a controlled voltage and frequency to the armature windings of the main generator and to provide field current to the main generator by way of the exciter so that motive power may be developed when the generator operates as a motor. For example, two separate inverters have been used in the past, one to provide electrical power to the main generator armature windings, and the other to provide electrical power to the exciter.
Once the engine is brought up to self-sustaining speed, the brushless, three-phase synchronous generator can be operated in its generating mode during which excitation current is provided to the exciter, and the main generator winding provides three-phase electrical output power.
Accordingly, the typical brushless, three-phase synchronous generator requires a rotor (i.e., armature) having windings and usually circuit components such as diodes. These windings and/or circuit components limit the speed with which the rotor can turn because, if the rotor turns too fast, the windings and/or circuit components may be ejected from the rotor due to centrifugal forces exerted by these rotating components, resulting in failure of the brushless, three-phase synchronous generator. Thus, for a given output power, an increase in rotating speed of an electrical machine requires that its size and weight be reduced.
On the other hand, induction machines, which use a squirrel cage rotor, avoid this problem because the squirrel cage rotor is solid, robust, light weight, and has no windings or circuit components. Therefore, it has been known to use induction machines as motors on aircraft. Induction machines, which have been used as generators, have only one stator winding. Moreover, it is generally thought that an induction machine operating as a generator must have its rotor driven above its synchronous speed, i.e., in a negative slip condition.
It is also generally thought that an induction machine cannot operate as a generator unless there is at least one synchronous generator available to excite it, and that an induction machine cannot supply its own excitation. However, some induction machines which have been used in the past as generators have relied upon a bank of capacitors connected across the stator winding by contactor switches in order to provide excitation current. The contactor switches control the current flow through the capacitors in order to control the excitation current. If the capacitive current drawn from the stator of the induction machine could be controlled properly and precisely, the induction machine could be self-excited in order to generate voltage at the same terminals where the capacitive current is drawn.
The present invention is directed to an induction machine that can be used as both a motor and a generator. The present invention is also directed to an arrangement permitting proper and precise control of the capacitive current drawn from the stator of the induction machine.
In accordance with one aspect of the present invention, an induction machine comprises a squirrel cage rotor, a main stator winding, an auxiliary stator winding, and a source of excitation current. The main stator winding and the auxiliary stator winding are magnetically coupled to the squirrel cage rotor. The source of excitation current is coupled to the auxiliary stator winding.
In accordance with another aspect of the present invention, an induction machine comprises a squirrel cage rotor, first, second, and third stator windings magnetically coupled to the squirrel cage rotor, first, second, and third capacitors coupled to the first, second, and third stator windings, respectively, and a solid state switch. The solid state switch is coupled to the first, second, and third capacitors, and the solid state switch is arranged to switch the first, second, and third capacitors so as to control excitation current supplied to the first, second, and third stator windings.
In accordance with yet another aspect of the present invention, a generator/starter system for starting an engine and for tapping power from the engine in order to generate electricity comprises a squirrel cage rotor, a main stator winding, an auxiliary stator winding, and a source of excitation current. The main stator winding is magnetically coupled to the squirrel cage rotor so as to provide an AC output in response to rotation of the squirrel cage rotor and so as to rotate the squirrel cage rotor in response to an AC input. The auxiliary stator winding is magnetically coupled to the squirrel cage rotor so as to excite the main stator winding. The source of excitation current is coupled to the auxiliary stator winding.